nca (Network Console on Acid) connects a pseudo
terminal to the consoles' virtual terminals
(/dev/tty0, etc.), allowing a remote user to read
and write directly to the console. It is supplied
with a version of OpenSSH that is hacked so that
one can connect to a Linux server and run nca very
early in the boot up sequence (immediately after
the root filesystem has been mounted read only).
It's the next best thing to a serial console for
fixing boot-time problems remotely.